By Michelle Goldsmith, Contributing Editor, Pump Industry Magazine
In the oil and gas industry, specialised pumps play a range of critical roles throughout the process of converting gas into LNG, storing it, and loading and unloading LNG tankers. Pumps that handle LNG are specialised and specified for the unique requirements of these applications, which require operation under cryogenic conditions, submersion of the pump unit, very high reliability and more. In this article, we take a closer look at some of the key requirements and considerations for LNG pumps.
To safely, efficiently and economically transport natural gas long distances, it is converted to liquified natural gas (LNG), which reduces it to around 1/600th of its original volume. Keeping LNG in its liquid form requires cryogenic conditions of at least -162ºC to be maintained throughout the transportation process.
The pumps that transport LNG, such as those that move it into, around, and out of the storage tanks on LNG tankers, must therefore operate at cryogenic temperatures. Due to the significant safety and economic consequences should equipment fail or leak LNG, the pumps used must be highly reliable, durable and constructed of compatible materials.
Pumps for LNG service
For LNG applications, specialised submerged motor-driven cryogenic pumps are used. Some of the benefits of submerged operation include that, as a cryogenic fluid, LNG is an excellent electrical insulator, and the motor is completely isolated from oxygen in the atmosphere, eliminating the risk of fire or explosion.
This design also removes the need for rotating seals, which can be problematic and introduce a potential point of failure. Submerged LNG pumps are constructed as a single unit, with the pump mounted on the same shaft as the motor, to be submerged in the LNG.
Some of the cryogenic LNG fluid moves through the gap between the pump’s rotor and stator to cool the motor. There are three main types used – fixed, vessel-mounted and retractable – with various adaptations available to suit the needs of a specific project and application.
For applications where a LNG pump is used in a piping system, it is mounted in its own suction vessel. The suction vessel takes the loads of the piping system and other structural elements, reducing the forces affecting the pump itself.
This design eliminates the need for alignment, as the pump bolts to the headplate of the suction vessel. Additionally, the vessel and liquid dampen the noise from the pump, and the pump can remain in the liquid, ready to operate whenever needed.
A retractable in-tank pump design allows the pump to be removed from the storage tank for maintenance while the tank itself continues to operate. These pumps are arranged in vertical pump columns, with a foot valve at the bottom, which prevents LNG from entering the column when the pump is removed.
The pump can be raised or lowered to open or close the foot valve without removing the headplate at the top of the column. The operators purge the column with nitrogen, and it can be safely removed at the top of the column. The only penetration from the pump to the outside atmosphere is the electrical leads to the motor.
Considerations for LNG pumps
Cryogenic LNG pumps incorporate unique features to meet the requirements of their specialised application. Some of the important requirements and considerations for pumping LNG include:
• Basic type – fixed, retractable, or vessel-mounted
• Construction materials
• Cooldown and warmup considerations
• Hydraulic considerations (e.g. operation at low NPSH)
• Handling considerations for pump size and weight
• Thrust equalising
• Bearing configuration
• Junction box assembly and electrical penetrations
• Vibration monitoring
• Adequate testing at the cryogenic temperatures and the power levels expected in service
Fixed, retractable, or vessel-mounted LNG pumps
As previously discussed, there are three basic types of submerged motor-driven LNG pumps. The exact configuration suited to an application depends on factors such as whether the pump potentially needs to be able to be removed from its tank for maintenance while the tank is still full of LNG, whether it is attached to a piping system, space considerations, and more.
Due to the contact between the submerged pump and the LNG, and the cryogenic application temperatures, LNG pumps (and associated equipment like valves) must be made of suitable materials for these conditions. The materials must adhere to applicable regulatory and industry standards to ensure safety.
Housings and rotating elements, such as impellers and inducers, are often fabricated from cast aluminium alloys, such as ASTM A356-T6, providing excellent thermal conductivity, stability and ductility at low temperatures, with a light weight (an important consideration for handling and on ships).
Austenitic stainless steel and wrought aluminium are also sometimes used for casings and other fixtures. Impellers are hard-coat anodised to provide erosion resistance. Shafts are frequently made from appropriate grades of stainless steel, chosen for ductility and strength at cryogenic temperatures, as well as electrical properties, as the motor rotor is installed directly on the shaft shared by the pump and motor.
Bronze is often used for wear rings and bushings, due to its thermal characteristics in the required temperatures. Other bushings options include graphite and sintered bronze materials. The bearings that support the rotor assembly are typically made of 440C steel (a martensitic high carbon 400 series stainless steel), providing corrosion resistance and temperature stability.
The electric pump motor and rotor are usually made from silicon steel laminated plates. The strength, light weight, and heat-transfer properties of this material are well suited to cryogenic applications.
Cooldown and warmup considerations
Before LNG can be taken on, the LNG tanks, lines and pumps must be pre-cooled to prevent stress on their materials from overly rapid cooling, or LNG converting to gas form. Likewise, LNG tank warmup is an important part of the process for removing gas prior to dry docking or when tanks must be prepared for inspection. The cooldown and start-up, as well as warmup, processes of the application should be considered in pump specification.
Like all pumps, to fulfill its duty, an LNG pump must be well-matched to the hydraulic requirements of the application. These include the required flow rates, suction pressure, and NPSH available. For in-tank applications, NPSHr (net positive suction head required) determines the usable and non-usable volume of an LNG tank.
The non-usable volume is that which remains in the tank due to the NPSHr limitations of the pump. Below the non-usable volume height, the pump will experience cavitation damage. If the pump is required to operate at low NPSH, high performance inducers and high-efficiency impellers are often required.
Handling considerations for pump size and weight
Pump size and weight have significant implications for ease of installation and maintenance, and pumps contribute to the total load on LNG tankers.
Thrust-equalising mechanisms balance thrust across the entire operating flow range, removing axial bearing loads. The system, which can be located either upstream or downstream of the motor, keeps the motor at discharge pressure, preventing vapour from forming in the motor housing.
Due to the poor lubricating qualities of cryogenic fluids, LNG pumps use specialised durable rolling-element bearings that can tolerate thrust in either direction and handle load reversals. Pumps associated with variable frequency drives (VFDs) may have ceramic ball bearings to prevent arcing due to eddy currents.
Junction box assembly and electrical penetrations
Electricity is connected to the submerged pumps through specialised junction boxes. As a penetration point between the LNG within the tank and the external atmosphere, the junction box and the electrical feed-through assemblies must be composed of appropriate materials and the design should incorporate various redundancies to prevent gas leakage to ensure safety.
Vibration monitoring systems alert operators to potential cavitation and operation outside set parameters, which could indicate the presence of vapour, meaning LNG is converting to natural gas.
Testing cryogenic LNG pumps
As a result of the unique design of LNG pumps and the operating temperatures required, they cannot be tested using water, and must be tested with LNG to verify performance as close to the conditions of actual use as possible. Testing with LNG close to design conditions verifies:
• Motor performance and starting ability in cryogenic liquid
• Mechanical alignment
• Pump performance characteristics using the cold boiling liquid, vital for NPSH testing
• No internal leakage due to deformation or differential shrinkage when the components are stressed at actual operating conditions
The unique requirements and safety-critical nature of LNG pumping applications mean that the pumps used must be specifically designed for the job and meet extremely high standards of durability and reliability.
As a result, a wide range of considerations go into specifying submerged motor-driven cryogenic LNG pumps for a project, ensuring that they can operate effectively in the extreme conditions required.
As the means of moving LNG between processing, storage and transport facilities, these pumps are instrumental to enabling LNG projects, allowing product to be transported safely and economically to markets.